Prior methods of formulating food products such as salad dressings, have used large quantities of an acid, such as acetic acid, to reduce microbial growth and, thus, produce a microbiologically stable product. For example, U.S. Pat. No. 4,352,832 describes the use of buffering salts and acetic acid to reduce the pH of a salad dressing product to render the dressing stable. However, the use of relatively high levels of acid tends to impact an undesirable acid "bite" to the food. It is also generally known that foods containing less water are easier to preserve, and exhibit longer shelf-life. Water in food exists in both "bound" and "free" forms. Bound water is not available to act as a solvent or to participate in chemical reactions and, thus, is not available to microorganisms for metabolic activity. Water activity (or a.sub.w) provides a measure of the availability of water for chemical reactions and microbial growth. Water activity is related to equilibrium relative humidity (ERH). The water activity of a solution defined in terms of ERH is as follows: ##EQU1## ERH may be determined using standard procedures such as the vapor pressure manometric technique.
Microorganisms have a minimum and an optimum a.sub.w for growth. The minimum a.sub.w at which growth can occur is important for determining the level for prevention of microbial growth in food. The minimum a.sub.w values for the growth of various microorganisms are known, for example, the minimum a.sub.w for the growth of most bacteria is from 0.90 to 0.91 a.sub.w. The addition of solutes such as various salts may be used to lower the a.sub.w.
In the past, knowledge in the art has dictated that foods with relatively high water/activity or pH (low acidity) require preservatives to prevent undesirable microbial growth, for example to prevent botulism. With the current trend toward reducing, and even eliminating the presence of preservatives in food products, there is a need for procedures for treating food products to reduce microbial growth without requiring preservatives, while maintaining a pleasing taste and consistency in the resulting food.